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java的Deque

Leetcode 77. 组合

题目：77. 组合
解析：代码随想录解析

解题思路

递归三部曲：递归函数的返回值以及参数；回溯函数终止条件；单层搜索的过程

代码

class Solution {List<List<Integer>> res = new ArrayList<>();List<Integer> paths = new ArrayList<>();private void backtracking(int n, int k, int startIndex) {if (paths.size() == k) {res.add(new ArrayList<>(paths));return;}for (int i = startIndex; i <= n; i++){paths.add(i);backtracking(n, k, i + 1);paths.remove(paths.size()-1);}}public List<List<Integer>> combine(int n, int k) {backtracking(n, k, 1);return res;}
}//减枝
class Solution {List<List<Integer>> res = new ArrayList<>();List<Integer> paths = new ArrayList<>();private void backtracking(int n, int k, int startIndex) {if (paths.size() == k) {res.add(new ArrayList<>(paths));return;}for (int i = startIndex; i <= n + 1 - (k - paths.size()); i++){paths.add(i);backtracking(n, k, i + 1);paths.remove(paths.size()-1);}}public List<List<Integer>> combine(int n, int k) {backtracking(n, k, 1);return res;}
}

总结

减枝是在每次循环开始的减枝；如果剩下的元素个数无法满足达到k个则减枝